Vitamin D is a well-known, fat-soluble hormone involved in mineral metabolism and bone growth. It is an essential positive regulator of calcium homeostasis. It can further dramatically facilitate intestinal absorption of calcium, although it also stimulates absorption of phosphate and magnesium ions. In the absence of vitamin D, dietary calcium is not efficiently absorbed at all and this leads to rickets. Increased parathyroid hormone levels are the first indication of rickets-associated vitamin D deficiency.
Vitamin D does not save significant biological activity. Rather, it must be metabolized within the body to the hormonally-active form, calcitriol (1,25-dihydroxycholecalciferol; 1,25-dihydroxy vitamin D3) with the following structure:
This transformation occurs in the liver, where cholecalciferol (vitamin D3) is hydroxylated to 25-hydroxycholecalciferol by the enzyme 25-hydroxylase; and in the kidney, where 25-hydroxycholecalciferol serves as a substrate for 1-α-hydroxylase, yielding 1,25-dihydroxycholecalciferol, i.e., calcitriol, the biologically active form of vitamin D. Calcitriol is a secosteroid that acts through binding to the VDR inside cells. VDRs have been suggested to reside in the cytoplasm and in the nucleus without hormone in an unbound state. The VDR binds several forms of cholecalciferol; however, its affinity for 1,25-dihydroxycholecalciferol is roughly 1,000 times that of 25-hydroxycholecalciferol.
By activating the VDR, calcitriol stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. VDRs are present in most, if not all, cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects beyond playing a role in mineral homeostasis and bone function.
Calcitriol, the active hormonal form of vitamin D, also acts through the VDR to regulate important functions, such as cellular proliferation and differentiation and immune functions. Calcitriol has biphasic effects on cell growth, where physiological doses stimulate cell proliferation, and high pharmacological doses inhibit cell growth. Calcitriol and its derivatives are thought to have utility in the treatment of cancers by retarding tumor growth, inducing apoptosis, and stimulating the differentiation of malignant cells. Current calcitriol derivatives are administered in large dosages to inhibit cancer growth. Unfortunately, such large dosages result in toxic levels of serum calcium. Further, the therapeutic possibilities of 1,25-dihydroxycholecalciferol are severely limited by the potent effect of this hormone on calcium metabolism, since serious side effects due to hypercalcernia will result from the high doses necessary to obtain a therapeutic effect on, for example, psoriasis, cancer or immunological disorders. In order to inhibit cell growth, current methodologies utilize combinations of vitamin D derivatives and therapies that specifically alleviate calcemic toxicities incurred by such high pharmacological dosages.
For example, 1,25-dihydroxycholecalciferol possesses potent anti-leukemic activity by inducing the differentiation of leukemia cells. Narvaez et al. (Endo. 137: 400-409 (1996)) disclose studies using EB 1089, which is a vitamin D analogue that induces breast tumor regression. Further, Colston et al. (Endo. 108: 1083-1086 (1981)) report anti-tumor effects of vitamin D derivatives. This study reports the presence of specific, high-affinity receptors for 1,25-dihydroxyvitamin D3 in malignant melanoma, and that in vitro administration of 1,25-dihydroxyvitamin D3 produces a marked increase in cell doubling time. Sato et al. (Tohoku J. Exp. Med. 138: 445-446 (1982)) report the utility of 1α-hydroxyvitamin D3 in in vivo experiments relating to the treatment of Sarcoma 180 and Lewis lung carcinoma implanted into mice. In these experiments, the vitamin D suppressed tumor growth, or inhibited pulmonary metastases. In another study, Disman et al. (Canc. Res. 47: 21-25 (1987)) disclose the utility of 1,25-dihydroxyvitamin D3 for inhibiting the growth of human colonic cancer xenografts in mice. Furthermore, Dokoh et al. (Canc. Res. 44: 2103-2109 (1984)) disclose the utility of 1,25-dihydroxyvitamin D3 on cultured osteogenic sarcoma cells.
Vitamin D and its analogues, while potentially useful in retarding abnormal cellular proliferation or tumor growth, have the disadvantage of being potent calcemic agents that cause elevated blood calcium levels by stimulating intestinal calcium absorption and bone calcium resorption. Accordingly, there has been a desire in the art for vitamin D analogues and derivatives having variant activities such that, for example, anti-leukemic activity is enhanced without concomitant enhancement of calcemic activity or toxicity. The present invention provides such analogues and derivatives. This and other objects and advantages, as well as additional inventive features, will be apparent from the detailed description provided herein.